Method for reacting gas mixtures by means of precious metal catalysts



' fll FIG. I. em 7 Dec. 24, 1940. G. M; CHASTAXN, JR 2,226,113

METHOD FOR HEACTING GAS MIXTURES BY MEANS OF PRECIOUS METAL CATALYSTS Filed Aug. 26, 193B 3 Sheets-Sheet l /ABSORPTION TOWER PLATINUM FILTER PLATINUM GAUZE COOLER couoeussns AM A MIXING CHAMBER A 7 HEAT 5 zxcmmssn NITRIC ACID INVENTOR GEORGE- CHASTAlN JR.

A ATTORNEY Dec. 24, 1940. r G. M. CHASTAIN, JR 2,226,113 METHOD FOR REACTING GAS MIXTURES BY MEANS OF PRECIOUS METAL CATALYSTS Filed Aug. 26, 1938 3 Sheets-Sheet 2 l8 y/1111100 2O \ls l8 M\ A 2| FIG.2

22-- 23- r {s X .v WA VA 22 I SECTION "A-A" INVENTOR GEORGE M. CHASTAIN JR.

BY M MA 9:.

ATTORNEY v FIG.4

I PLATINUM FILTER 6l% Dec. 24, 1940. G. M. CHASTAIN, JR 2,226,113

METHOD FOR REACTING GAS MIXTURES BY MEANS OF PRECIOUS METAL CATALYSTS Filed Aug. 26, 1938 :s sheets-sheets /ABSORPTION TOWER I PLATINUM FILTER PLATINUM GAUZE' AMMONIA conozussas I MIXING CHAMBER |5' 'COOLER CONVERTOR I0 I I l l 6 I HEAT 7 EXCHANGER I l 'l l4 a 8 NITRIC ACID INVENTOR GEORGE m. CHASTAIN JR.

Patented Dec. 24, 1940 UNITED STATES PATENT OFFICE METHOD FOR REACTING GAS MIXTURES BY MEANS OF PRECIOUS METAL CATALYSTS Application August 26, 1938, Serial No. 228,888

7 Claims.

This invention relates to a method of reactin gas mixtures by means of a precious metal catalyst, and more particularly to a method of recovering precious metals lost from such catalysts during such operations.

The oxidation of ammonia, for instance, involves the mixing of ammonia gaswith oxygen or oxygen-containing gases, e. g., air, and the passage of such gas mixture through or over a 10 catalyst maintained at an elevated temperature, whereby the ammonia is oxidized to oxides of nitrogen. The oxides of nitrogen are primarily used in the manufacture of nitric acid, by absorption of said oxides of nitrogen in water, but

they are also used in the manufacture of sulfuric acid.

The catalyst is usually in the form of gauze,

but may also consist of perforated plates, strips or other suitable structures. The catalyst metal of which the structure is formed comprises a precious metal, usually platinum, or an alloy thereof with another precious metal, e. g. an alloy of platinum and rhodium or of platinum and iridium, and may also contain non-precious metals or materials.

The catalysts are operated at elevated temperatures, such as 650 C. to 1000" C. and above, and suffer considerable metal losses during the operation, due to physical and chemical attack by the gases. The amount of catalyst metal thus lost increases with increasing temperature and increasing rate of production, and some catalysts suffer greater metal losses than others due to 3 difference in abilityto withstand the attack by the gases.

Such losses represent a substantial momentary loss, due to the high cost of the catalyst metal. For instance, processes of oxidizing ammonia 4 to oxide of nitrogen involve operation at relatively high temperatures and at a relatively high rate of production and the metal losses therein encountered are often as high as and even higher than 2.2 troy ounces per 100,000 lbs. avoirdupois 45 of ammonia burned.

I have found that a very substantial proportion of such catalyst losses otherwise going to waste may be simply and readily recovered by the-use of a suitable filter in the gas lines beyond the re- 50 action chamber, without introducing the expected complications of obstruction to gas flow or pressure due to obstruction in the pipes, even at the high temperatures encountered in such gas streams. I am able so to recover the catalyst 55 otherwise lost, no matter whether the process be operated at atmospheric pressure or at a pressure above or below atmospheric pressure.

I find that my filter is surprisingly eflicient in its recovery of the lost catalyst, when it is considered that the particles of the catalyst are 6 carried in the gas stream in a very fine state of subdivision and are distributed throughout a very large volume of gas. While I do not confine myself to any particular theory of operation,

I believe that the high efliciency of my filter is 10 due to the fact that the catalyst particles carried by the gas are electrostatically charged and adhere to the filter bed because of this charge, rather than merely being mechanically retained. This theory of operation is believed to be supl5 ported by the fact that a majority of the catalyst particles are retained near the surface of the filter-bed with which the gas stream first comes into contact, with a smaller proportion retained by the deeper layers of the filter bed. 20

In the manufacture of nitric acid the filter is placed in theconduit leading from the oxidation chamber and prior to the point at which the water vapor condenses, as it has been found that the catalyst metal, e. g. platinum, which is con- 25 tained in the gas stream in very finely divided form will, at least partly, dissolve in the liquid acid which begins to form at the condensation temperature of the water vapor, and as the catalyst metal may otherwise become partly deposited in the pipes of for instance the cooler-condensers.

It is desirable to place the filter as close as possible to the oxidation chamber, considering temperature and construction of the apparatus, in order to catch the catalyst metal before substantial quantities thereof have had an opportunity to become deposited in any pipes. If the filter is thus placed relatively close to the oxidation chamber it is desirable to provide for at least one more filter, placed subsequent to the first 4o filter, in order to catch any fine particles that may have passed through the first filter.

The filter itself is comprises of glass wool, quartz wool, rock wool, fibrous asbestos, porous alundum, porous porcelain, or the like. The filter is preferably arranged in a chamber in such a way that no material pressure is interposed against the passage therethrough of the gases.

If filter materials capable of withstanding high temper tures of 650 C. and more, such as quartz wool, a1 ndum, and the like are used, it is possible to place the filter below the catalyst and actually inside or at the exit of the converter.

7 While I may use a variety of different refractory material, as a filter in my process, I have found that fibrous materials are particularly suited to the purpose since they-have less tendency to obstruct the fiow of the gas stream'than materials in the form of, for example, granules or pellets. I have found glass wool and quartz wool entirely'satisfactory for this p se.

Asspecific illustrations of two alternate embodiments of my invention, reference is made to the accompanying drawings, in which Figure 1 represents a fiow sheet of a process of manufacture of nitric acid utilizing one embodiment of my invention, and Figure 2 represents a section of a specific embodiment of a filtering device adapted for use in accordance with my invention, and Figure 3, a section of the device of Figure 2 along the lines A-A', and Figure 4 represents a flow sheet of a process of manufacture of nitric acid utilizing an alternate embodiment of my invention.

Referring now specifically to Figure 1 of the accompanying drawings, i represents a conduit, through which is passed ammonia gas, to mixing chamber 2, where it is mixed with air entering via conduit 3, heated by heat exchange 4, and passing via pipe 5 to said mixing chamber 2. From mixing chamber 2- the thoroughly mixed gas, consisting of ammonia and air, passes directly to oxidation chamber 6, where it passes through the hot catalyst I maintained at a high heatby the exothermic reaction. The gases, after passing the catalyst, consist mainly of water and oxides of nitrogen, and pass via conduit 8 through heat interchanger 9 and conduit III, to filter H, preferably adapted to withstand relatively high heat (about 300? C.) and not materially obstruct gas, fiow, as herein set forth, then via conduit l2 to cooler-condensers l3, in series, where a substantial proportion of the water is condensed, then via conduit M to absorption tower I 5, where the nitric oxide gases are converted into 'a nitric acid of about 61% concentration,

which runs via conduit iii to suitable storage tanks (not shown).

Referring now specifically to Figure 2 of the accompanying drawings,'the entire device represents the filter shown as l l in Figure l. The gases coming from the oxidation chamber enter the filter via opening I! and pass into the housing l8, which may be constructed of aluminum, stainless steel, or other metal resistant to nitric acid, then pass through filter bed l9, composed of glass wool, where the very fine particles of catalyst carried by the gas stream are caught and retained, the filtered gases then passing into chamber 20 and to outlet 2|, where they meet conduit i2 of'Figure 1 and pass on to the rest of the process.

Referring now specifically to Figure 3 of the accompanying drawings, which represents a section of the filter of Figure 2, along the lines AA, i9 represents the filter bed, 22 a perforated plate of Monel metal, or other metal resistant to nitric acid, 23 a wire screen of like metal, 24 a metal plate of like metal and having somewhat larger openings than plate 23.

It will be noted that the filtering device of Figure 2 is contained in a housing comprising several parts, bolted or clamped together (not shown), in a manner such that the whole can be readily disassembled for removing and replacing the filter bed.

It will be further noted that the filtering d vice of Figure 2 provides a filter bed l9 having a surface area several times the cross-sectional area of the entering conduit I! or' of the exit monia gas to mixing chamber 2' where it is mixed with air entering via conduit 3', heated by heat exchanger 4, and passing via pipe 5' to said mixing chamber 2'. From mixing chamber 2' the thoroughly mixed gas, consisting of ammonia and air, passes directly to oxidation chamber I.

where it passes through the hot catalyst 1 maintained at a high heat by the exothermic reaction. The gases, after passing the catalyst, consist mainly of water and oxides of nitrogen, and pass via conduit 2 to filter 25, adapted to withstand high temperature (about 700 C.) and not materially obstruct the gas fiow. This filter 25 may be constructed, for example, as shown in Figures 2 and 3. From the filter 25 the gases pass through heat interchanger 9' and conduit It to a second filter 26, preferably adapted to withstand relatively high temperature (about 300 C.) and not materially obstruct gas flow, as hereinbefore set forth, then via conduit l2 to cooler condensers H, in series, then via conduit It to absorption tower l5 where the nitric oxide gases are converted in nitric acid of about 61% concentration, which runs via conduit IE to suitable storage tanks (not shown).

When the filter has been in operation for a considerable time, the exact length of time depending on the quantity of gas converted, the condition of the catalyst, the temperature of the catalyst and the rate of production, it is removed, the filter bed removed and replaced with a fresh filter bed, and operations resumed.

It is also possible of course to have a second filter arranged in parallel to the first filter through which the gas stream can be diverted when the first filter requires removal; in this way no interruptions whatever in the oxidation operation take place.

The removed filter bed is then subjected to suitable treatment to dissolve out therefrom the precious metal content, which is then recovered from such solution in any suitable manner customarily used for the recovery thereof from solutions containing the same. If the catalyst comprises platinum or an alloy thereof, the filter may, for instance, be submitted to a treatment with aqua regia which will dissolve the platinum and also most of the other metals of the platinum group which are present in finely divided state. The solution of aqua regia is then evaporated; the nitroso compounds ofthe platinum are destroyed by repeated evaporation with hydrochloric acid; finally the platinum solution is filtered and the platinum is precipitated with ammonium chloride as (NHUzPtCle which is ignited and transferred into platinum sponge. Other precious metals which may have gone in solution are precipitated with zinc and the concentrates are refined. If small amounts of rhodium, iridium,

nually from the catalyst in the normal operation of converting very large quantities of gases, e. g. ammonia, the recovery of a considerable part of such loss represents a very substantial saving in the operations.

While I have described my invention in particular reference to the oxidation of ammonia to oxides of nitrogen and to the oxidation of ammonia to nitric acid, it is evident that my invention also applies to the recovery of precious metals lost from precious metal catalysts used in the reacting of other gas mixtures and the conversion of gases other than ammonia provided the precious metal catalyst is operated at elevated temperatures. One such other reaction process is for instance the reaction at a high temperature in the presence of a catalyst of the type hereinbefore described of a mixture containing ammonia, hydrocarbons, e. g. methane, and oxygen, to produce hydrogen cyanide or hydrocyanic acid.

Also I do not restrict my invention to the particular construction shown, but, as is evident, I may use any suitable shape or form of apparatus suitable for filtrations of hot, corrosive gases, which does not materially obstruct the flow of said gases.

The term catalyst particles" as used hereafter refers to the catalyst lost as hereinbefore described.

This application is a continuation-in-part of my application Serial No. 185,635, filed January 19, 1938.

What I claim and desire to protect by Letters Patent is:

1. A method of recovering precious metal lost during a gaseous reaction in the presence of a precious metal catalyst which includes passing the gases through a self-supported, precious metal, catalytic mass, and then passing the reacted and unreacted gases through a porous filter constructed of a mass of discrete particles of a material which is refractory at filtering temperatures, said filter positioned at a point where the temperature is maintained above the condensation temperature of any material in the gas stream which is condensable to liquid at normal temperatures and pressures, and recovering the catalyst particles retained within the said porous filter.

2. A method of recovering precious metal lost during a gaseous reaction in the presence of a precious metal catalyst which includes passing the gases through a self-supported, precious metal, catalytic mass, and then passing the reacted and unreacted gases through a porous filter constructed of a mass or fibrous material which is refractory at filtering temperatures, said filter positioned at a point where the temperature is maintained above the condensation temperature of any material in the gas stream which is condensable to liquid at normal temperatures and pressures, and recovering the catalyst particles retained within the said porous filter.

3. The method of recovering precious metal lost during a. gaseous reaction in the presence of precious metal catalyst which includes passing the gases through a self-supporting, precious metal. catalytic mass, and then passing the reacted and unreacted gases through a porous filter contractory at filtering temperatures, said filter postructed of glass wool, said filter positioned at a point where the temperature is maintained above the condensation temperature of any material in the gas stream which is condensable to liquid at normal temperatures and pressures, and recovering the catalyst particles retained within the said porous filter.

4. The method of recovering precious metal lost during a gaseous reaction in the presence of precious metal catalyst which includes passing the gases through a self-supporting, precious metal, catalytic mass, and then passing the reacted and unreacted gases through a porous filter constructed of quartz wool, said filter positioned at a point where the temperature is maintained above the condensation temperature of any material in the gas stream which is condensable to liquid at normal temperatures and pressures, and recovering the catalyst particles retained within the saidporous filter.

5. The method of recovering precious metal lost during a gaseous reaction in the presence of precious metal catalyst which includes passing the gases through a self-supporting, precious metal, catalytic mass, and then passing the reacted and unreacted gases through a plurality of porous filters constructed of masses of discrete particles of a material which is refractory at filtering temperatures, said filters positioned at a point where the temperature is maintained above the condensation temperature of any material in the gas stream which is condensable to liquid at normal temperatures andpressures, and recovering the catalyst particles retained within the said plurality of porous filters. 1

6. A method of recovering precious metal lost during the oxidation of ammonia to nitric acid by means of a precious metal catalyst which includes passing a gaseous mixture containing ammonia and oxygen through a self-supported, precious metal, catalytic mass, and then passing the reacted and unreacted gases of said gaseous mixture through a porous filter constructed of a mass of discrete particles of a material which is resitioned at a point where the temperature is maintained above the condensation temperature of any material in the'gas stream which is condensable to liquid at normal temperatures and pressures, and recovering the catalyst particles retained within the said porous filter.

7. A method of recovering precious metal lost during the oxidation of ammonia to nitric acid by means of a precious metal catalyst maintained at a temperature of between about 650" C. and about 1000 C., which includes passing a gaseous mixture containing ammonia and oxygen through a self-supported, precious metal, catalytic mass, and then passing the reacted and unreacted gases of said gaseous mixture through a porous'filter constructed of a mass of discrete particles of a material which is refractory at filtering temperatures, said filter positioned at a. point where the temperature is maintained -above the condensation temperature of any material in the gas stream which is condensable to liquid at normal temperatures and pressures, and recovering the catalyst particles retained within the said porous filter.

' GEORGE M. CHASTAIN, Jn. 

